The Project Owner is the account that's financially responsible for all Project fees. Ownership can be transferred to another Project Admin, as long as the admin has an active Pro account and an up-to-date credit card on file.
Project Ownership is denoted by a Crown icon. In the Project dropdown menu, a crown will appear next to the projects you own. In the Groups/Users list, the crown will appear next to the name of the Project Owner.
To transfer ownership and financial responsibility:
Step 1: Select the desired project from the Project dropdown menu
Step 2: Go to the Groups/Users section
Step 3: Mouse over the user you would like to transfer ownership to and open the More menu next to their name. 
Note: If the user isn't already in the project, click the 'Add New User' button in the bottom left to add them before continuing. Make sure the user has a Pro account.
Step 4: If the user isn't already an admin, from the 'More' menu select 'Make a Project Admin.'
Step 5: Once the user has admin status, open the 'More' menu again and select 'Make Project Owner.'
Step 6: Confirm that the transfer information is correct and click 'Transfer.'
Step 7: When the transfer is confirmed the new Owner will be moved to the top of the list and the crown icon will be moved to their name. If the name is not moved to the top of the list, refresh the page.
